Appropriate breathing requires 1) feedback concerning the level of CO2 from central chemoreceptors, and 2) a tonic 'drive', partly from CO2, and partly from other sources including the rostral ventrolateral medulla (RVLM). Recent work established that 1) central chemoreception is present at many brainstem locations, and 2) the retrotrapezoid nucleus (RTN) is a key RVLM site that provides both chemoreception and a tonic drive to breathe. We ask: Why are there so many central chemoreceptor sites? How do they work? What is the physiological role of the RTN in the control of breathing? We will evaluate chemoreceptor and RTN function during sleep and wakefulness in a chronic unanesthetized rat model using a microdialysis probe to deliver substances to the RTN (or other site). The probe tip is 1 mm in length and 240 mum in diameter, a volume of 45 nl. It allows repeated application of neuroactive substances at the same site in the same animal with continuous measurement of ventilation and oxygen consumption (whole body plethysmograph), arousal state (EEG, nuchal EMG), body temperature and blood pressure (telemetry), and, in some cases, blood gases and pH. Approximately 2/3 of the experiments use this model; 1/3 an anesthetized, ventilated rat with phrenic activity as the measure of respiratory output. For studies of chemoreception physiology, we produce focal tissue acidosis by CO2 microdialysis in both models. For studies of mechanism, we alter neural function by injection/dialysis within the focal region of acidosis in the anesthetized rat. For studies of the RTN, we inhibit neurons reversibly by dialysis with muscimol, a GABA-A receptor agonist, in the chronic model. Central chemoreceptor physiology is significant; CO2 is a key component of the respiratory control system and CO2 retention in disease causes morbidity. Chemoreception and RTN function vary with arousal state, and thus are likely to be important in sleep disordered breathing, and the RTN is hypothesized to be an animal homologue for the arcuate nucleus, described as abnormal in SIDS victims.